Conventional inkjet recording methods include the continuous method and the drop-on-demand method. According to the former method, continuously-ejected inks are selectively caused to strike a medium. According to the latter method, on the other hand, inks are selectively ejected. In recent years, however, drop-on-demand printers have become the mainstream. These drop-on-demand printers can be divided into two types, one being the bubble jet system that inks are rapidly heated to eject them by the resulting bubbles, and the other being the piezoelectric system that inks are ejected using ceramics (piezoelectric elements) which deform when voltages are applied.
The above-described piezoelectric system, which uses conventional oil-based inks, ejects the inks by using piezoelectric elements as pumps to convert electric energy into mechanical energy, and basically, can eject various ink materials. However, this system ejects inks depending on the existence or non-existence of signals. At end faces of nozzles through which the ejection of the inks is suspended, the solvents in the inks evaporate. As a consequence, ink clogging may take place by the deposition of solid components from the inks; or the ejection of the inks may be inhibited by increased viscosity of the inks within the nozzles due to concentrated inks. It has, accordingly, been required to frequently conduct maintenance work. In particular, inks which are useful in the production of prints for outdoor use employ, as printing media, non-absorbent media, namely, plastic films such as polyvinyl chloride sheets. They, therefore, use as an ink solvent a solvent having relatively high volatility. As a consequence, the inks dry up rapidly, resulting in marked occurrence of ink clogging.
When the air dissolved in an ink exceeds a certain level, microbubbles which occur in printhead nozzles as a result of the development of cavitations by high-frequency oscillations of piezoelectric elements absorb pressures, leading to a reduction in drive response. In addition, ejection troubles may be induced by the production of bubbles within nozzles as a result of changes in the surrounding air temperature.
With a view to overcoming the above-mentioned problems, inkjet pigment inks making use of high-boiling solvents have been proposed as substitutes for conventional oil-based inks (JP-A-2003-96370). Glycol ether esters employed as the above-mentioned high-boiling solvents are high in boiling point and low in vapor pressure, so that the proposed inks do not cause much printhead nozzle clogging during printing. Compared with low-boiling solvents which have been used conventionally, the glycol ether esters are, however, lower in polymer solubility. When a commercially-available solid polymer is used as a fixing polymer by dissolving it in a solvent or when a polymer having a high glass transition point is used, the resulting ink is, therefore, not provided with ejectability such as ink ejection or ejection recovery to any sufficiently satisfactory level. Especially when employed in such piezoelectric inkjet printers as mentioned above, no sufficient ink ejectability is available. Further, when printing is resumed subsequent to a temporary discontinuation, the ink ejection recoverability is lowered, thereby failing to provide prints of stable quality.
Therefore, an object of the present invention is to provide an oil-based ink composition for inkjet printers, especially inkjet printers of the piezoelectric inkjet printers, which is excellent in ejection stability, ejection recoverability and storage stability and is also superb in drying properties on recording media and in the abrasion resistance of prints.